872 PHYSIOLOGY OF DIGESTION AND SECRETION. 



baceous material is accumulated to form the vernix caseosa. The 

 well-known uropygal gland of birds is homologous with the mam- 

 malian sebaceous glands, and its secretion has been obtained in 

 sufficient quantities for chemical analysis. Physiologically it is 

 believed that the sebaceous secretion affords a protection to the 

 skin and hairs. Its oily character doubtless serves to protect the 

 hairs from becoming too brittle, or, on the other hand, from being 

 too easily saturated with external moisture. In this way it prob- 

 ably aids in making the hairy coat a more perfect protection against 

 the effect of external changes of temperature. Upon the surface of 

 the skin, also, it forms a thin, protective layer that tends to prevent 

 undue loss of heat from evaporation of the sweat and possibly is 

 important in other ways in maintaining the physiological integrity 

 of the external surface. 



Excretion of C0 2 . In some of the lower animals the frog, 

 for example the skin takes an important part in the respiratory 

 exchanges, eliminating CO 2 and absorbing O. In man, and pre- 

 sumably in the mammalia generally, it has been ascertained that 

 changes of this kind are very slight. Estimates of the amount of 

 CO 2 given off from the skin of man during twenty-four hours vary 

 greatly, but the amount is small, about 7 to 8 gms. in twenty-four 

 hours, unless there is marked sweating, in which case the amount is 

 noticeably increased, 



